They need it to weed out one-hit wonders.
The bulk of their money comes from blockbuster sellers that draw in casual readers but with pbooks they have no way of telling whether people buying the book actually read it and like it enough to buy a follow-up. Whether they've found another Harry Potter or another Goldfinch.
They need it to adjust the print run and marketing commitment as well as in negotiating contract terms. (If half the copies of a million seller never get finished, the follow-up isn't very likely to sell a million. It probably won't get to half a million.)

How much it matters was exposed last month with all the whining and FUD surrounding the pay out changes at Kindle Unlimited from tradpub authors who *aren't* in KU and never have been. A lot of established tradpub authors owe big chunks of their income to books that never get read so this is something that can evolve into a wedge issue in tradpub.